In its essential elements, a photodetector may be defined as an instrument which detects radiation by absorbing photons and generating an observable or detectable electric current or electronic charge. These devices can be classified generally as either photoconductive or photoemissive. In the latter, photons are absorbed in an appropriate photoelectric material which emits electrons into a vacuum. When accelerated under a potential or electric field, such emitted electrons are collected on an anode and the current or electric charge may be measured. More sophisicated devices may employ the photoemitted electrons to eject yet more electrons by causing them to collide with another electron emissive material thereby generating a higher current and achieving a gain in the total number of emitted electrons.
The photoconductive device, however, functions on a different principle in that a photoconductive material is laminated between a conducting semitransparent electrode and another electrode. When an electric field is applied across the electrodes and the laminate absorbs photons, carriers are generated leading to a change of the resistance of the device. The current is then measured using an appropriate circuit.
Each of the described techniques forms the basis for imaging devices. The photoemissive type of device may be easily damaged by excessive intensity of light energy. Conventionally, such devices may be constructed so as to include a mechanical iris or stop which may be either automatically or manually adjusted to obtain acceptable exposure levels. The iris employed for this purpose is basically similar to those which are widely used in photographic cameras, for example.
These types of imaging tubes and imaging devices are extensively employed in military systems for reconnaissance, surveillance, intrusion detection, and other imaging functions designed to detect extremely small differences in levels of radiant light energy in predetermined spectral bands. The exposure of such imaging devices to sudden high intensity levels of irradiance can severely damage the photocathode surfaces or saturate the system to a point rendering it totally or temporarily useless. The danger of such damage is most likely when a sudden occurrence of such excessive illumination takes place faster than the mechanical response time of the automatic iris. Bright flash lamps, flares, pulsed lasers, and nuclear explosions are among some of the sources of such highly undesirable sudden, excessive illumination.
Accordingly, there is a need for an automatic shutter which is high-speed in operation so as to overcome the deficiency of slowness of operation of the mechanical type iris or shutter which customarily may take several milliseconds to operate from its fully opened to its fully closed condition.